Gas separation modules are commonly used to selectively separate a particular gas from a gas mixture. Two of the most common gas separation modules are polymer membranes and metallic composites. Polymer membranes can provide an effective and cost-efficient option for separating a gas at low temperatures. Where separations must be performed in conjunction with high-temperature processing, however, polymer membranes are generally unsuitable because they tend to thermally decompose.
The development of high-temperature processing, along with tighter environmental regulations, requires utilization of gas separation modules that provide high flux, high selectivity of separation, and the ability to operate at elevated temperatures. Instead of polymers, metallic composite modules can be employed to serve these needs. A composite gas separation module can consist of a metallic membrane having selective gas permeability mounted on a porous substrate.
An area of high-temperature gas separation that is of particular interest is the separation and purification of hydrogen gas from a reaction gas mixture. A composite module for selectively separating hydrogen gas at high temperatures can include a palladium (Pd) membrane. Ideally, the palladium membrane is permeable to hydrogen but not to other gases. When hydrogen gas (H2) contacts the membrane, the hydrogen molecules dissociate and hydrogen atoms diffuse into the membrane. Accordingly, hydrogen can selectively pass from a surrounding atmosphere through the palladium membrane. The selectively separated hydrogen atoms then reassociate into H2 gas and pass into a volume on the opposite side of the module.
The effective life of a typical composite gas separation module having a hydrogen-selective metal membrane bonded to a porous substrate often is limited by diffusion of substrate components into the membrane which decreases the hydrogen permeability of the membrane. The rate of diffusion of the substrate components is greatest when the substrate is at or above its Tamman temperature. A metal lattice at its Tamman temperature is subjected to considerable thermal (atomic) vibration. If there is an interface between two metals, such thermal vibration significantly increases the mobility of metal atoms and their consequent diffusion. The Tamman temperature of a material is equal to one-half of its melting point temperature in Kelvin. For example, in the case of a hydrogen-selective palladium membrane on a stainless steel substrate, palladium and stainless steel have melting point temperatures of 1552° C. (1825 K) and 1375–1400° C. (1648–1673 K), respectively. The corresponding Tamman temperatures are about 640° C. (913 K) and 550–560° C. (823–833 K), respectively. The lower of these Tamman temperatures determines the temperature where a significant increase in intermetallic diffusion can occur. Accordingly, at temperatures around 550° C. considerable thermal vibration and diffusion of stainless steel substrate components into a palladium membrane can be expected in such a composite gas separation module. The alloy created by the diffusion of stainless steel substrate components into a palladium membrane can have reduced hydrogen permeability.
One solution to this problem has been to use a ceramic substrate, which tends to exhibit less diffusion of substrate components into the hydrogen-selective metal membrane than a predominantly metallic substrate. However, ceramic substrates are typically more brittle than predominantly metallic substrates. Further, ceramic substrates can be more difficult to fabricate and also can be more difficult to join to other components in a gas separation system.
Gas separation modules formed purely of a hydrogen-selective metal such as palladium also have been used. Eliminating the presence of the substrate in such a gas separation module can remove the problem of intermetallic diffusion. However, such a module can be very expensive to produce and can lack the mechanical strength that can be required for high pressure and/or high temperature applications. For example, a gas separation module formed purely of a hydrogen-selective metal generally must have a much greater thickness than a composite gas separation module to provide adequate mechanical strength. This increase in thickness can reduce the gas flux that can be established through the module.
Therefore, a need exists for composite gas separation modules (and methods for their fabrication) that overcome or minimize the above-referenced problems.